Momentum dependence of pseudogap and superconducting gap in variation theory

To consider the origin of a pseudogap and a superconducting (SC) gap found in the high-T_c cuprates, we evaluated the momentum dependence of the singlet gap corresponding to the pseudogap and the SC gap in the t–J model, using an optimization variational Monte Carlo (VMC) method. In the underdoped regime, the singlet gap is significantly modified from the simple d_x²-y²(d)-wave gap (∝ cos k_x − cos k_y) by the contribution of long-range pairings. Its angular dependence at the quasi Fermi surface is qualitatively consistent with those experimentally observed in both hole and electron-doped cuprates. On the other hand, a SC gap is almost unchanged, preserving the original simple d-wave form. Thus, it seems that the incoherent part of the singlet gap mainly influences the forms of observed gaps.     

Lateral restorable characteristics of the high-Tc superconducting maglev vehicle above the permanent magnet guideway

The lateral restorable characteristics of a translational symmetry high-T_c superconducting maglev system are investigated by measuring its resonant frequency (f_RF) after a lateral displacement. The difference between whether this lateral displacement is restorable, meaning elastic or inelastic, is determined by whether or not the maglev body returns to its original position after a lateral displacement. The maximum restorable lateral displacement (δ_MRLD) is determined by the sudden change of the f_RF vs. the maximum lateral displacement (δ_MLD) curve. The f_RF of the high-T_c superconducting maglev system with different field-cooling height (FCH) and working height (WH) was obtained from the frequency domain vibration curve which was measured by a vibration measurement system. The results showed that, the δ_MRLD was reduced when the WH was decreased. The maximum restorable guidance force (F_MRGF) was found to not always increase with the lowering of the WH for the same FCH. The lateral restorable stiffness (k_LRS) was always enhanced with the decrease of the WH. The decrease of the δ_MRLD with the WH is interpreted by the fact that, the tangential field component (ΔH) across the surface of the high-T_c superconductor (HTSC) is easier to exceed the J_cλ value (J_c is the critical current density and λ is the London penetration depth) when the WH is lowered, and this makes the trapped flux lines become more susceptible in escaping its pinning sites.     

On the preparation and the characterization of superconducting thin films of YBa2Cu3O7−x

The results of our initial efforts to deposit thin films of YBa₂Cu₃O_7−x system on sapphire substrate are described. The deposited films are shiny black in appearance and are of quite uniform chemical composition. The annealed films exhibit zero resistance superconducting transition temperatureT _c(R=0) ranging between 23 K and 30 K.     

Effect of antiferromagnetic ordering on temperature dependent superconducting gap in ErNi2B2C: Laser-photoemission spectroscopy

We have performed temperature (T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi₂B₂C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results, we find a sudden deviation from the BCS prediction just below T_N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy.     

Band structure,Fermi surface,and superconducting gap in FeAs-based superconductors revealed by angle-resolved photoemission spectroscopy

In this paper, we present a brief review on our angle-resolved photoemission measurements on the band structure, Fermi surface, and superconducting gap of the newly-discovered FeAs-based high temperature superconductors. (1) The Fermi surface of the FeAs-based compounds are characterized by the hole-like Fermi surface sheets near Γ (0, 0) and the existence of singular Fermi spots near M(π,     

Scanning tunneling spectroscopy and break junction spectroscopy on iron-oxypnictide superconductor NdFeAs(O0.9F0.1)

Iron-oxypnictide superconductor NdFeAs(O_0.9F_0.1) was studied using both low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and tunnel break junction (BJ) methods. STM topography showed granular and spot structures with a typical size of several nanometers, most probably governed by fluorine atom distribution. The majority of STS conductance, G, versus voltage, V, curves revealed V-shaped structures, whereas some of G(V) dependences possessed coherent gap peaks or kinks at gap energies. At the same time, G(V) dependences obtained by the BJ technique showed clear-cut coherence peaks with peak-to-peak distances V_pp = 4Δ/e ∼ 25 mV at 4.2 K, where Δ is the superconducting energy gap, e > 0 is the elementary charge. This yields Δ(0) = 6–7 meV, so that the ratio 2Δ(0)/k_BT_c is about 3–4, k_B being the Boltzmann constant. This value is consistent with the conventional weak-coupling s-wave Bardeen–Cooper–Schrieffer theory.     

Giant Nernst effect in the pseudogap phase of high Tc superconductors

We have investigated theoretically the Nernst effect in unconventional (d-wave) charge and spin density waves (UDW). In the presence of magnetic field, Landau levels are formed, and the gapless behaviour of the low energy excitations change into gapped behaviour. When additional electric field is applied, the quasiparticles drift with a velocity of E × B/B², and carry entropy. From this, the Nernst coefficient can be calculated using the Kelvin relation. The present results account very nicely for the measured Nernst signal in the pseudogap phase of high T_c superconductor La_2−xSr_xCuO₄ and Bi₂Sr_2−yLa_yCuO₆. This indicates that the large Nernst effect is a clear signiture of UDW.     

The effect of lattice strain and antiferromagnetism on superconducting gap in cuprates

The doping of the parent insulating cuprate systems destroys long range antiferromagnetic order and develops superconductivity accompanied by Jahn–Teller type lattice distortion in copper 3d electrons in CuO₂ plane. In the present communication, we propose a microscopic model to study the interplay of these long range orders. The superconducting gap (Δ), staggered magnetic field (h) and lattice strain (e) are calculated from the Green’s functions and solved self-consistently for different sets of model parameters of the system. Finally the temperature dependent density of states helps to calculate the magnitudes of the order parameters.     

Quasiparticle dissipation in the measurement process of phase qubit using high- superconductors

We discuss how to make use of high-T_c d-wave Josephson junctions in the construction of a phase qubit. We especially focus on the effect of the quasiparticle dissipation and the zero energy bound state on the macroscopic quantum tunneling which corresponds to the final measurement process of the d-wave phase qubit.     

Iron doping effect on superconducting properties of MgB2

Iron-doped MgB₂ bulks are prepared by hybridized diffusion method using nano-powder and macro-powder of pure iron as iron source. The doping effect on superconductivity transition temperature, T_c, and critical current J_c have been investigated. It is found that both T_c and J_c of MgB₂ show quite different features depending on the particle size of the dopant powders. It is demonstrated that different from iron bulk or large size powders, iron nano-powders are active dopant for MgB₂ which suppresses both T_c and J_c of MgB₂.     

Global and local critical current density in superconducting SmFeAsO1−xFx measured by two methods

The critical current densities of polycrystalline bulk SmFeAsO_1−xF_x prepared by the powder-in-tube (PIT) method and by a conventional solid-state reaction were investigated using the remnant magnetic moment method and Campbell’s method. Two types of shielding current, corresponding to global and local critical current densities J_c were observed using both measurement methods. The global and local J_c were on the order of 10⁷ A/m² and 10¹⁰ A/m² at 5 K, respectively. The local J_c decreased slightly with increasing magnetic field. The global J_c was independent of the preparation method, while the local J_c was larger for samples prepared by PIT than for those prepared by solid-state reaction.     

Magnetic gap effect on the tunneling conductance in a topological insulator ferromagnet/superconductor junction

The tunneling conductance on the surface of a topological-insulator-based ferromagnet/superconductor (F/S) structure is studied where S is an s-wave superconductor with superconducting order parameter ∼Δ. The conductance is calculated based on the BTK formalism. The magnetization in F is applied along the z-direction () in order to induce the energy-mass gaps (m) for the Dirac electrons in the F-region. In this work, the influence of energy gap due to the magnetic field in the F-region on the conductance is emphasized. The Fermi energy mismatch between F (EFF=EF) and S (EFS=EF+U), where the gate potential U is applied to the electrode on top of S, is also considered. As a result, a biased voltage V can cause the conductance switch at eV=Δ, depending on the value of the magnetic field. The conductance is found to be linearly dependent on either m or U. The slope of the curve can also be adjusted. This linear behavior in a topological-insulator-based F/S structure may be valuable for electronic applications of the linear-control-current devices. The tunneling conductances of the quasi-Dirac-particle in a topological-insulator-based F/S junction are quite different from those of a graphene-based F/S junction.     

Optical properties and thermal stability of LaYbO3 ternary oxide for high-k dielectric application

A new ternary rare oxide dielectric LaYbO₃ film had been prepared on silicon wafers and quartz substrates by reactive sputtering method using a La-Yb metal target. A range of analysis techniques was performed to determine the optical band gap, thermal stability, and electrical property of the deposited samples. It was found the band gap of LaYbO₃ film was about 5.8 eV. And the crystallization temperature for rapid thermal annealing (20 s) was between 900 and 950 °C. X-ray photoelectron spectroscopy results indicate the formation of the SiO₂ and silicate in the interface between silicon wafer and LaYbO₃ film. The dielectric constant is about 23 from the calculation of capacitance-voltage curve, which is comparable higher than previously reported La₂O₃ or Yb₂O₃ film.     

Effect of post-deposition annealing on structural and electrical properties of high-k HoTiO3 gate dielectrics

In this article, the authors developed a high-k HoTiO₃ gate dielectric deposited on Si (1 0 0) through reactive cosputtering. They found that the HoTiO₃ dielectrics annealed at 800 °C exhibited excellent electrical properties such as high capacitance value, small density of interface state, almost no hysteresis voltage, and low leakage current. This phenomenon is attributed to the decrease in intrinsic defect (related to oxygen vacancy) due to a rather well-crystallized HoTiO₃ structure and composition observed by X-ray diffraction, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy, respectively.     

Higher harmonics in the voltage on a superconducting wire carrying AC electrical current

The problem of determining the harmonic content in the voltage that appears on a superconducting wire carrying cosine-like AC current was resolved theoretically, using two approaches. First, the Fourier components of the voltage spectrum were found by numerical integration. Importance of individual terms was established, leading to two conclusions: a) it is the cosine component of the 3rd harmonic that represents the bulk of harmonic distortion, b) for the practical purposes it is sufficient to consider higher harmonics with n ≤ 7. Then, the analytical formulas were derived. While for the sine components a general expression containing an infinite series was found, closed-form formulas were derived for the cosine components of the harmonics 1, 3, 5, 7. Consequences of the results to the experimental technique used to study the AC transport properties of superconductors are discussed.     

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

Asymmetric superconducting gap and DDW state in high-Tc cuprates

The asymmetric gap superconductivity is considered in orthorhombic high T_c cuprates. Recent experiments predict an anisotropy in the gap where |Δ(0,π)|> |Δ(π,0)| and the gap node deviates from the diagonal direction toward the k_x axis. The temperature dependencies of the specific heat and penetration depth along the a and b directions are calculated for the anisotropic gap superconductors. However, the anisotropy in the penetration depth can be consistent with the experimental observations only after the inclusion of the plane and chain coupling. The d-density wave (DDW) phase that explains the pseudogap has also been considered to study the phase diagrams of the cuprates.     

Effect of bond length and radius on superconducting transition temperature for FeAs-based superconductors

By comparing the data of lattice parameters of more than 50 new FeAs-based high temperature superconductors in two syetems, the effect of bondlength on superconducting transition temperature (T C ) was found that, for both FeAs superconductor systems with similar ionic radii of cation A, the bond length L As-A between Arsenic atom and its nearest neighbor cation at the A site is in an inverse proportion to T C , i.e. the larger the bond length L As-A , the lower the T C . In addition, we also found a notice...     

Terahertz time-domain reflection spectroscopy for high-Tc superconducting cuprates

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation, a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry out the polarized reflectivity measurements. Using our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La_2−xSr_xCuO₄ single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T_c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra.     

Low-temperature growth of high-Tc superconducting films by plasma-enhanced metal-organic chemical vapor deposition

Plasma-enhanced MOCVD in which metal-organic compounds are sublimated directly into the growth chamber is studied for the first time as a new low-temperature process for growing superconducting YBa₂Cu₃O-_-x thin films. Y(THD)₃, Ba(THD)₂, Cu(THD)₂ and oxygen are used as metal sources and oxydizing agent. Emission spectroscopy reveals that activated metal-organic compounds and activated oxygen species are present during film growth. Superconducting YBa₂Cu₃O_7-x films whose zero-resistivity temperature are 50 K and 82 K are grown at 550°C and 600°C.